Amongst the many, many posts on here about the BFG letters turning brown, there was one where a chemist explained it all very well. That it was a chemical reaction and to turn them white again, you needed the proper solvent/cleaner, etc, etc to reverse the reaction. He tested his theory and found he was correct.
I have BFG's on my show pony 71 Demon. They are about 3 years old now. I wouldn't call the letters brown but they were certainly not as white as they should be. So after reading that post (probably 1-2 years ago), I followed his advice and got out a scotchbrite pad and acetone. A little scrubbing with the pad and a wipe with a rag with acetone and viola, white letters. I've found the white lasts about 3-5 months and then repeat the process. Give it a try!
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One other thing that I have not tried is "back in the day," my dad had me wash his car and "do the whitewalls." That meant get the Brillo pad out and start scrubbing. I always hated that job. But it sure worked well back then to turn the whitewalls white and chew up my fingers! But haven't tried it on these BFG white letters so no clue if it does anything.
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